linux-kernel - Re: [PATCH v8 6/9] usb: dwc3: qcom: Add multiport controller support for qcom wrapper

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Date:   Thu, 8 Jun 2023 11:42:29 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Krishna Kurapati PSSNV <quic_kriskura@...cinc.com>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Felipe Balbi <balbi@...nel.org>, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org, quic_pkondeti@...cinc.com,
        quic_ppratap@...cinc.com, quic_wcheng@...cinc.com,
        quic_jackp@...cinc.com, quic_harshq@...cinc.com,
        ahalaney@...hat.com
Subject: Re: [PATCH v8 6/9] usb: dwc3: qcom: Add multiport controller support
 for qcom wrapper

On Thu, Jun 08, 2023 at 01:21:02AM +0530, Krishna Kurapati PSSNV wrote:
> On 6/7/2023 5:07 PM, Johan Hovold wrote:

> > So there at least two issues with this series:
> > 
> > 	1. accessing xhci registers from the dwc3 core
> > 	2. accessing driver data of a child device
> > 
> > 1. The first part about accessing xhci registers goes against the clear
> > separation between glue, core and xhci that Felipe tried to maintain.
> > 
> > I'm not entirely against doing this from the core driver before
> > registering the xhci platform device as the registers are unmapped
> > afterwards. But if this is to be allowed, then the implementation should
> > be shared with xhci rather than copied verbatim.
> > 
> > The alternative that avoids this issue entirely could indeed be to
> > simply count the number of PHYs described in DT as Rob initially
> > suggested. Why would that not work?
> > 
> The reason why I didn't want to read the Phy's from DT is explained in 
> [1]. I felt it makes the code unreadable and its very tricky to read the 
> phy's properly, so we decided we would initialize phy's for all ports 
> and if a phy is missing in DT, the corresponding member in 
> dwc->usbX_generic_phy[] would be NULL and any phy op on it would be a NOP.

That doesn't sound too convincing. Can't you just iterate over the PHYs
described in DT and determine the maximum port number used for HS and
SS?
 
> Also as per Krzysztof suggestion on [2], we can add a compatible to read 
> number of phy's / ports present. This avoids accessing xhci members 
> atleast in driver core. But the layering violations would still be present.

Yes, but if the information is already available in DT it's better to use
it rather than re-encode it in the driver.
 
> > 2. The driver is already accessing driver data of the child device so
> > arguably your series is not really making things much worse than they
> > already are.
> > 
> > I've just sent a couple of fixes to address some of the symptoms of
> > this layering violation here:
> > 
> > 	https://lore.kernel.org/lkml/20230607100540.31045-1-johan+linaro@kernel.org/
> >
> 
>   As you pointed out offline to me that using xhci event notifiers I 
> mentioned on [3], if it is not acceptable to use them as notifications 
> to check whether we are in host mode, I believe the only way would be to 
> use the patches you pushed in.

I still think we'll end up using callbacks from the xhci/core into the
glue driver, but dedicated ones rather than using usb_register_notify().

The fixes I posted can work as a stop-gap solution until then.

> > Getting this fixed properly is going to take a bit of work, and
> > depending on how your multiport wake up implementation is going to look
> > like, adding support for multiport controllers may not make this much
> > harder to address.
> > 
> > So perhaps we can indeed merge support for multiport and then get back
> > to cleaning this up.
> So, you are referring to use the patches you pushed today as a partial 
> way to cleanup layering violations and merge multiport on top of it ? If 
> so, I am fine with it. I can rebase my changes on top of them.

Right. A bit depending on how your wakeup implementation looks like, it
seems we can merge multiport support and then address the layering
issues which are already present in the driver.

> [1]: 
> https://lore.kernel.org/all/4eb26a54-148b-942f-01c6-64e66541de8b@quicinc.com/
> [2]: 
> https://lore.kernel.org/all/ca729f62-672e-d3de-4069-e2205c97e7d8@linaro.org/
> [3]: 
> https://lore.kernel.org/all/37fd026e-ecb1-3584-19f3-f8c1e5a9d20a@quicinc.com/

Johan